PRE Commissioning Procedures Tr Rev01 PGCIL

POWER GRID CORPORATION OF INDIA LTD. PRE-COMMISSIONING PROCEDURES / ूी किमशिनंग ू बयाएं DOC NO: D-2-01-03-01-02- SECOND REVISION / तीय संशोधन

PRE-COMMISSIONING PROCEDURES FOR SUBSTATION

INTRODUCTION Energization of the system will be made step by step in zones as small as possible bound by the disconnectors installed in the system. Energisation is the first time connection of the Substation equipment to the high voltage system. It is to be done stepwise in the smallest possible zones (subsystems) as permitted by the design (position of disconnectors). During the energisation, only the very small excitation currents will flow. Only very little power will be required to supply the corona losses and the no load losses of the equipment. Overall procedure, safety rules, dispatch procedures, switching sequences, observations, passing criteria and documentation of results are treated in this document. Once all the zones have been energized in sequence, the energisation is concluded with an 8 hour charging to ascertain the capability of the system. The detailed instructions/test procedures for each zone to be energized will be given in separate section of this documentation. The contents of this document are as following: 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 1.0

Subsystem limits Overall procedures Auxiliary system Protective system Dispatch procedures Switching sequences Energisation De-energisation Observations and duration Passing criteria Documentation SUBSYSTEM LIMITS Main Circuits: • • • • • • • • •

AC equipment: AC Switch yard, Control and Protection Station auxiliaries: Station auxiliary power Telecommunications (PLCC) Air Conditioning and Ventilation Telephone system (Exchange) Illumination Fire protection Others 1


2.0

OVERALL PROCEDURE Instructions for the work and supervision are taken care of by the test leader. However, all switching and operational activities will be executed by the regular operators incharge under the IE Rules. First it is to be ascertained that the subsystem to be energized is ready for operation and has been properly handed over (released) in writing. This will include all safety aspects, protections, control systems, auxiliaries and final inspection. Then the subsystem is prepared according to switching order, halted at the last step before closure of the ultimate Circuit Breaker. Before proceeding, the status is given a final check including confirmation with the dispatch centre i.e. CPCC/RLDC. Thereafter, energisation is executed by closure of the Circuit Breaker. Inspection as well as instrument readings are to be taken. Successive subsystems are then energized without special urgency but also without unnecessary delay. When the whole system has been energized, including the AC line, it will be kept in this state for 8 hours or more for observation with continuous inspection and monitoring. However, recommendation of the Manufacturers may be checked.

3.0

SAFETY PROCEDURES Energisation implies an abrupt and serious change of working conditions of the plant/ system. In order to avoid serious accidents, thorough information must be circulated to all the personnel and rigorous discipline enforced. This should include a statement that trespassing of safety barriers, even if accident does not occur, must be cause of immediate dismissal and removal from the plant area. All equipment tests and pre-commissioning tests must have been completed, terminated and documented. The subsystem must be formally declared ready for energization and handed over for operation in writing. The subsystem must be bound by warning signs and Safety barriers according to the IE Rules. Guards shall be posted in suitable positions. The subsystem must have been thoroughly cleaned and all non-regular objects (ladders, tools, vehicles, work-benches, debris etc) removed. It is necessary that the Commissioning in-charge (test leader) takes a final, thorough inspection of all areas 2


involved to ascertain that this has really been executed. No deviations whatsoever are to be tolerated. This is necessary in order to maintain the discipline and the respect for safety rules. 4.0

AUXILIARY SYSTEM All auxiliary systems needed for regular operation of the unit must be declared tested and formally in writing handed over for operation. A thorough inspection by the test leader for the confirmation of the readiness of the auxiliary system shall immediately precede the energization.

5.0

PROTECTIVE SYSTEM Before energization it must be ascertained that all protective systems for the unit to be energized are operative. This includes confirmation that the protections have been properly tested and that the tests have been documented. It also includes verification by inspection or otherwise, if necessary by repetition of trip test, that the protections are actually functionally enabled. This verification serves to prevent that energization takes place of a unit where a protection has been disabled for test or other reason.

6.0

DISPATCH PROCEDURES All operational activities (switching, power changes, changes in redundant reserves and alike) must be coordinated and communicated with the system dispatcher i.e. CPCC/RLDC.

7.0

SWITCHING PROCEDURES For each activity the instructions to the operators and the communications to the dispatchers will be made in writing or by confirmed telephone messages. The format established by POWERGRID for switching orders will be followed. When preparing the switching orders for the first energization, the only assumption permitted is that nothing is known about the status of the system. The implication of this is that each and every activity must be listed and described so that complete and unambiguous conditions become the result. Examples of such switching orders will be part of the following instructions for the different subsystems. Every attempt will be made to get them complete and self contained. However, local conditions may deviate from those assumed, they shall be mistrusted and considered 3


suspect at the best as guiding examples. They must be scrutinized in detail and double checked before application. 8.0

ENERGIZATION Execution of the energization is simply the last event in the switching sequence, switching of the close control button for the relevant Circuit Breaker.

9.0

DE-ENERGIZATION Instructions about de-energization are to be given only if this is part of the test. Otherwise de-energization will be considered part of regular operation.

10.0

OBSERVATIONS AND DURATION Visual and audible inspection (look and listen) of the relevant equipment and reading of permanent instrumentation will be made. The system should be charged at least for 8 hours. During this time continuous monitoring and inspection will be maintained in control room, auxiliary systems areas and switchyard.

12.0

PASSING CRITERIA Neither insulation breakdown nor protective system actions must occur. No irregular equipment behaviour noise, vibration, high temperature is permitted. Corona discharges may not be “unreasonable”. Local discharges that may be attributable to sharp points shall be carefully located and recorded. After termination of the energisation the equipment shall be closely inspected and the points rounded or covered. No unscheduled change of system or of equipment is permitted during the 8 hour energized condition.

13.0

DOCUMENTATION Switching and operational activities will be recorded in regular manner in the operators log. Likewise all readings of permanent instruments. Copies of this log, notes on special observations from inspections and other measurements will constitute the test records.

4


TESTING AND COMMISSIONING PROCEDURES FOR BAY EQUIPMENT INTRODUCTION All the equipment after erection/assembly at site, should be tested in order to check that it has not been damaged during transport, erection/assembly to such an extent that its future operation will be at risk. The significance of various tests with brief procedure has been elaborated in the subsequent sections of this document. Regarding the detailed testing methods / procedures for conducting various pre-commissioning tests refer to Doc. No. D-2-03-XX-01-01: Maintenance Procedures for Switchyard Equipments-Part 1: EHV Transformers & Reactors and Doc No. D2-03-XX-01-01 –Part 2: Other Switchyard Equipments. The instruction manuals of various equipments being used for the test shall also be consulted in case of any difficulty in operation of the testing instrument. The results of the test shall be documented on the test record formats as mentioned below, which are also part of this documentation: Sl. No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

FORMAT NO. No. CF/CT/01/R-2 DATED 01/08/07 No. CF/CVT/02/R-2 DATED 01/08/07 No. CF/BAY/03/R-2 DATED 01/08/07 No. CF/SA/04/ R-2 DATED 01/08/07 No. CF/CB/05/ R-2 DATED 01/08/07 No. CF/ICT/06/ R-2 DATED 01/08/07 No. CF/SR/07/ R-2 DATED 01/08/07 No. CF/ISO/08/ R-2 DATED 01/08/07 No. CF/WT/09/ R-2 DATED 01/08/07 No. CF/C&P/10/R-2 DATED 01/08/07

EQUIPMENT CURRENT TRANSFORMER CVT BAY/FEEDER SURGE ARRESTOR CIRCUIT BREAKER ICT SHUNT REACTOR ISOLATOR/GROUND SWITCH WAVE TRAP Control & Protection

These formats have all the possible test recordings to be performed at site before energization. NOTHING SHOULD BE TAKEN FOR GRANTED AND NO CONNECTION SHOULD BE ASSUMED TO BE CORRECT UNTIL IT HAS BEEN TESTED AND PROVED. A Commissioning team should be constituted as per procedures laid down in Works & Procurement Policy and Procedures (Clause 2.15.4.6&7 of WPPP Vol. II), which shall oversee/ coordinate with erection agency/ manufacturer for the pre-commissioning test and charging of the equipment The pre-commissioning tests after erection shall be carried out in accordance with latest technical specifications issued by CC/Engg., Pre-Commissioning procedures / formats issued by CC/OS and Guidelines/ instruction from the Equipment Manufacturer. (Clause 2.15.4.8(iv) of WPPP vol. II) The test results shall be checked by Commissioning team. After all pre-commissioning checks and tests are found to be acceptable taking into account permissible deviation limits, the commissioning team, in consultation with Regional O&M shall give clearance for charging. (Clause 2.15.4.8 (viii) of WPPP vol. II). In case of deviations in commissioning test results beyond permissible limit as per guidelines/ standards in vogue, the commissioning team shall 5


submit its report along with recommendations to the concerned Site In-charge. In such an event, the test charging will be carried out only after obtaining approval of Head of O&M of the Region based on a proposal of execution site in consultation with Regional OS. (Clause 2.15.4.8 (ix) of WPPP vol. II). Please Note: It is preferable to have manufacturer representative during charging and get the charging clearance from equipment manufacturer after review of the test results by them. The test records had to be signed by the responsible personnel from the OEM, the supplier, the erection agency and the POWERGRID representative. The test formats/records are to be distributed as follows: 1.

Regional O&M office

2.

Corporate Operation Services

3.

Corporate Engineering

4.

Manufacturer/ Supplier of the equipment

5.

Erection Agency.

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1.0

TRANSFORMER AND REACTOR

1.1

Following points to be checked After Receipt of transformer / reactor at Site:

1.1.1

N2 pressure and Dew point to be checked after receipt of transformer at Site. It should be within permissible band (as per graph provided by manufacturer & given below)

1.1.2

The data of impact recorder shall be analyzed jointly in association with the manufacturer. In case the impact recorder indicates some serious shocks during shipment, further course of action for internal inspection, if necessary shall be taken jointly.

1.1.3

Oil Samples shall be taken from oil drums/ tanker received at site and sent to our Lab for oil parameter testing. The copy of test certificate of routine testing at oil refinery should be available at site for comparison of test results.

1.1.4

Unpacking and Inspection of Accessories to be carried out taking all precautions so that the tools used for opening do not cause damage to the contents. Fragile instruments like oil level gauge, temperature indicators, etc. are to be inspected for breakage or other damages. Any damaged or missing components should be reported to equipment manufacturer, so that the same can be investigated or shortage made up as per the terms/ conditions of the contract.

1.1.5

Core Insulation Test shall be carried out to check insulation between Core (CC&CL) and Ground.

Figure -1 : Graph depicting variation of N2 gas pressure w.r.t temperature 7


1.1.6

1.1.7

After receiving the accessories at site same should be inspected and kept ready for immediate erection: •

If erection work can not start immediately due to some reasons, then accessories should be repacked into their own crates properly and packing list should be retained.

•

All packings should be kept above ground by suitable supports so as to allow free air flow underneath. The storage space area should be such that it is accessible for inspection, water does not collect on or around the area and handling/transport would be easy. Immediately after the receipt of main unit and also the accessories, same should be inspected and if found satisfactory, the unit should be erected completely and filled with dry transformer oil as per the instruction.

•

It is preferable to store the main unit on its own location/foundation. If the foundation is not likely to be ready for more than three (3) months, then it should be stored at suitable location preferably filled with oil. Fill the oil as per instructions up to tank oil level mark. Fill the gap above oil with dry air and fit silica gel breather on any opening on the cover, with suitable pipe. The transformer preferably be provided with oil conservator including oil-level indicator and breather.

•

If the transformer is to be stored up to three (3) months after arrival at site, it can be stored with N2 filled condition. N2 pressure and dew point to be monitored on daily basis so that chances of exposure of active part to atmosphere are avoided.

•

In case of storage of transformer in oil-filled condition, the oil filled in the unit should be tested for BDV and moisture contents once in every three months. The oil sample should be taken from bottom valve. If BDV is less and moisture contents are more than as given for service condition then oil should be filtered.

During erection, the active part of transformers should be exposed to atmosphere for minimum time and either dry air generator should be running all the time or dry air cylinders may be used to minimize ingress of moisture. The transformer should be sealed off after working hours. It is practical to apply a slight overpressure overnight with dry air or N2 inside – less than 300 mbar ( 30 kPa or 0.3 atmospheres). Next day the pressure is checked and suspected leaks may be detected with leak detection instruments , with soap water or with plastic bags tightened around valves (being inflated by leaking air) TABLE 1- Variation of Dew Point of N2 Gas Filled in Transformer Tank w.r.t Temperature For transformers with a gas pressure of 2.5- 3 PSI, the acceptable limits of dew point shall be as under: (Source: Courtesy BHEL, Bhopal) Temperature of Insulation in °F 0 5 10 15 20

Maximum Permissible dew point in °F -78 -74 -70 -66 -62

Temperature of Insulation in °C -17.77 -15.0 -12.22 -9.44 -6.66

Maximum Permissible dew point in °C -61.11 -58.88 -56.66 -54.44 -52.22 8


25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 110 120 130 140

1.2

-58 -53 -48 -44 -40 -35 -31 -27 -22 -18 -14 -10 -6 -1 +3 +7 +16 +25 +33 +44

-3.33 -1.11 +1.66 +4.44 +7.44 +9.99 12.77 15.55 18.33 23.11 23.88 26.66 29.44 32.22 34.99 37.75 43.33 48.88 54.44 59.99

-49.99 -47.22 -44.44 -42.22 -39.39 -37.22 -34.99 -32.77 -29.99 -27.77 -25.55 -23.33 -21.11 -18.33 -16.11 -13.88 -8.88 -3.88 +0.55 +5.55

Insulating Oil When oil is dispatched to site separately it is usually in sealed steel drums. The oil to be used to filling and topping up must comply with IS 335 (latest revision). Before oil is accepted samples have to be taken and checked with respect to dielectric strength and dielectric losses. The latter is important, since dirty transportation vessels can significantly contaminate the oil. High dielectric losses cannot be removed by filter treatment, such lots have to be rejected. If the oil is supplied in railroad or trailer tanks, one or two samples are sufficient. If the oil is delivered in 200 litres drums, the following scheme for checking is recommended. Number of drums delivered 2 to 5 6 to 20 21 to 50 51 to 100 101 to 200 201 to 400

No. of drums to be checked 2 3 4 7 10 15

1.2.1 Samples from Oil Drum Check the seals on the drums. The drum should first be allowed to stand with bung (lid) vertically upwards for at least 24 hours. The area around the bung should be cleaned & clean glass or brass tube long enough to reach to within 10mm of the lower most part of the drum should be inserted, keeping the uppermost end of the tube sealed with the thumb while doing so. Remove the Thumb thereby allowing oil to enter the bottom of 9


the tube. Reseal the tube and withdraw an oil sample. The first two samples should be discarded. Thereafter, the sample should be released into a suitable receptacle. 1.2.2 Parameters of Transformer Oil The oil as received at site for filling and topping up in the transformer must comply with POWERGRID specification for acceptance criteria. The oil sample from the transformer tank, after filling in tank before commissioning should meet the following specifications as per IS: 1866 – 2000 (latest Revision) given in table-2 below: Table-2 Sr. No. 1. 2. 3. 4. 5. 6. 7 8. 9. 10. 11. a. b. 12.

Property Breakdown Voltage (BDV) (kV rms)-Min Moisture content –(ppm) Max Dielectric Dissipation Factor (Tan δ) at 90 ºC and 40-60Hz -Max Resisitivity at 90 ºC-Min ( x 1012 ohm-cm) Interfacial Tension (mN/m) -Min Viscosity at 27ºC-Max – cSt Density at 29.5ºC gm/cm3 - Max Flash point ºC – Min Pour point ºC – Max Neutralization Value mg KOH/gm of oil –Max Oxidation Stability of Uninhibited Oil Neutralization Value mg KOH/gm of oil –Max Sludge percent by mass – Max Oxidation Stability for Inhibited Oil

Highest Voltage of Equipment <72.5 kV 72.5 – 170 kV >170 kV 40 50 60 20 15 10 0.015 0.015 0.01 6 35 27 0.89 140 -6 0.03 0.4 0.1

6 35 27 0.89 140 -6 0.03

6 35 27 0.89 140 -6 0.03

0.4 0.4 0.1 0.1 Similar values as before filling

1.3 Oil Filling (For detailed Oil filling procedure, please refer to Appendix-2 of Doc. No. D-2-03-XX-01-01: Maintenance Procedures for Switchyard Equipments-Part 1: EHV Transformers & Reactors) Introduction: For transformer dispatched with gas filled from the work, the filling of oil inside the tank is done under vacuum. Transformer of high voltage ratings and their tanks are designed to withstand partial or full vacuum. Manufacturer’s instructions should be followed regarding the creating of full or partial vacuum during filling the oil in the tank. Final tightness test with vacuum (i.e. leakage test or Vacuum Drop Test): Before the oil filling is started, a final check is made of the tightness of the complete transformer with its cooling equipment by applying vacuum. When any vacuum is applied to a transformer, empty of oil, a leakage test must be carried out to ensure that there are no leaks on the tank which would result in wet air being drawn into the transformer. The procedure is as follows: •

Connect the vacuum gauge to a suitable valve of the tank. 10


• • • • • • •

Connect the vacuum pump to another opening. Evacuate the transformer/ reactor tank until the pressure is below 3 mbar ( 0.3 kPa or about 2 mm of Hg) Shut the vacuum valve and stop the pump. Wait for an hour and take a first vacuum reading – say P1 Take a second reading 30 minutes later- say P2 Note the quantity of oil, V tonnes according to the rating plate The leakage rate for any transformer can be calculated from the formula:

Leak rate (mbar litre /sec) = (Main tank oil volume in litres)* (drop in vacuum i.e. change in pressure in mbar)/ Time in seconds The leak rate observed at various points of time should not exceed the limits as given below: Time in hours After 1 hour End of 3rd –4th hour End of 7th –8th hour End of 11th –12th hour

Allowable leak rate in mbar litre/ sec 100 75 50 25

If the leak rate is high, it indicates excessive leakage, then all gasketted joints should be checked for any leakage. As soon as 25 millibar-litre /sec leak rate is achieved, re-start the vacuum pump, open the valve and continue pumping until the required vacuum (1.3 mbar or less) is reached and maintained for the specified period. This vacuum must be maintained for forty-eight hours before and during the subsequent oil filling operations by continuous running of the vacuum pumps. When filling a transformer with oil it is preferable that the oil be pumped into the bottom of the tank through a filter press or other reliable oil drying and cleaning device should be interposed between the pump and the tank. In case the transformer is provided with an On Load Tap Changer (OLTC), while evacuating the main transformer tank, the diverter switch compartment may also be evacuated simultaneously so that no undue pressure is allowed on the tap changer chamber. While releasing vacuum, the tap changer chamber vacuum should also be released simultaneously. For this one pressure equalizer pipe should be connected between main tank and tap changer. Manufacturer’s instruction manual should be referred to protect the air cell/diaphragm in the conservator during evacuation. The oil must pass from the oil tanker, or settling tanks, through an oil conditioning plant to the top oil filter valve and the oil must conform to the quality requirements as given in POWERGRID- Transformer Oil Specification. The oil flow at the entry valve must be controlled to maintain a positive pressure above atmospheric and to limit the flow rate if necessary to 5000 litres / hour, or a rise in oil level in the tank not exceeding one metre / hour (as measured on the oil level indicator)

11


Continue oil filling until the level reaches approximately 200 mm above the ambient oil level indicated on the magnetic oil level gauge in the expansion vessel. Then, release the vacuum, with dry air of dew point -40deg C or better. The diverter tank can now be topped up at atmospheric pressure. Reconnect oil outlet hose to valve on flange on tap changer diverter head. Reinstate breather and very slowly top up the diverter switch such that the correct level is reached in the diverter expansion vessel. In the event the expansion vessel is overfull drain oil from flange into a suitable container until the correct level is reached. When the vacuum filling of the transformer and diverter tank is complete, the cooling system/ Radiator bank can be filled (WITHOUT VACUUM) at atmospheric pressure, via an oil processing plant. Oil must be admitted, very slowly, through the bottom cooler filter valve, with the cooler vented at the top and the top cooler filter valve unblanked and open to atmosphere. As the oil level reaches the vents and open valve, close off and shut down the processing plant. Note: Care must be taken not to pressurise the coolers/ radiators. Upon completion, open the top cooler isolating valve in order to equalise the pressure in the cooler with the transformer tank. This will also allow contraction or expansion of the oil as the ambient temperature changes. Before oil filling into conservator inflate the air cell to 0.5 PSIG i.e. 0.035kg/cm2 max. slightly by applying pressure (N2/Compressed dry air) so that it can take shape. Release pressure. Fit breather pips but do not fit breather in position, instead put wire mesh guard over and flange of the pipe. This will ensure free air movement from the air cell to the atmosphere. Use flow meter / indicator on outlet of filter machine and regulate the flow using the valve to limit oil filling rate to 2000 litres per hour max. in case filter capacity is more. Push oil slowly into conservator through the transformer via valve No. 5 (valve 2,3 & 4 to remain open) till the oil comes out first through valve Nos.2 & 3 (close these valves) and then through valve No. 4. Allow some oil to come out through valve No.4. Oil should come out freely into the atmosphere. This will ensure that oil air inside the conservator is expelled out and the space surrounding the air cell is full of oil. (Close valve No. 4). During all these operations valve No.1 to remain closed. Drain oil from the conservator by gravity only through valve No. 1 or through drain valve of the transformer via valve No. 5. Do not use filter machine for draining oil from the conservator. Also do not remove buchholz relay and its associated pipe work, fitted between the conservator and the transformer tank while draining oil. Stop draining oil till indicator of magnetic oil level gauge reaches position-2 on the dial, which is corresponding to 30°C reading on the oil temperature indicator.

12


Figure-2 : General Arrangement For Oil Conservator

1.4

Drying of Transformer/ Reactor

The drying of a new Transformer/ Reactor required when the moisture get absorbed by the solid insulation used in Transformers/ Reactors due to various reasons. The process of drying out a transformer is one requiring care and good judgment. If the drying out process is carelessly or improperly performed, a great damage may result to the transformer insulation through over heating, etc. In no case shall a transformer be left unattended during any part of the dry out period. The transformer should be carefully watched through out the dry-out process and also observations shall be carefully recorded. When the transformer is dried out it is necessary to ensure that fire fighting equipment is available near the transformer. Various methods can be adopted for drying out a transformer depending upon the facilities available/provided at site.

13


1.4.1 DRYING OUT OF TRANSFORMER / REACTOR BY HOT OIL CIRCULATION USING HIGH VACUUM OIL FILTER MACHINE 1.4.1.1 PURPOSE

When the equipment is exposed to atmosphere during erection, atmospheric moisture is absorbed by the solid insulation. This moisture in the paper will reduce the dielectric strength and the ageing will be faster than the normal. The purpose of this procedure is to remove the moisture from the transformer / reactor when water is absorbed in the cellulose. When the equipment is opened and the winding insulation is exposed to atmosphere, it is a must that it should be dried out. Please Note that this procedure is applicable only for low wet equipments and for moderate or high wet equipments, procedure given in section 1.4.2 to be followed. 1.4.1.2 PRECAUTIONS:

1. Rubber hoses should not be used for oil since the sulphur in the rubber would react with oil. 2. If the transformer / reactor is fitted with air cell in the conservator, the conservator should be isolated while doing oil filtration / degassing. 3. At any point of time it is seen that the oil temperature should not be more than 60°C, otherwise the property of the oil gets deteriorated due to oxidation. 5. It is to be taken care that there is no mixing of parafinic or naphthenic oil. If the machine is already used for other type of oil, the machine needs to be flushed with 50 -100 litres of the same oil for which the machine is to be used. 6. The manual of the filter plant needs to be read thoroughly before starting the operation and all the additional precautions stipulated by the manufacturer of the plant needs to be taken into account. 7. Similarly the particular transformer / reactor manual also to be read carefully and all additional precautions mentioned need to be taken into consideration. 8. Coolers are to be isolated during drying process. 9. It is to be ensured that the tank will withstand the full vacuum. If not vacuum needs to be regulated according to the manufacturer's instruction. 1.4.1.3 PREPARATION

* For drying out operation of transformer / reactor the coolers are to be isolated so as to prevent heat loss during the process. * The transformer / reactor tank has to be completely logged with a tarpaulin or insulating cover to avoid heat loss. 14


* It is advisable to provide electric heaters at the bottom as well as at the sides of the transformer / reactor tank outside but within the insulation cover so that the process is speeded up. * A motorised megger (5KV) to be connected to the HV terminals to ground in case of reactor or HV + LV to ground in case of transformer for measurement of insulation resistance during the drying out. * The oil level in the equipment should be just above the winding and a vacuum gauge to be connected at the top of the transformer / reactor to read the applied vacuum. 1.4.1.4 PROCEDURE

Drying out a transformer using an oil filtration plant is the most practical method of drying at site compared to all other methods. In this method the drying out is achieved by circulating hot oil through a stream line filter. Connect the inlet of the filter machine to the bottom of the transformer / reactor tank. Connect the outlet to the top. Connect one vacuum pump to the top of the tank after lowering the oil level slightly to just above the winding. Run the oil filtration plant as well as the vacuum pump connected to the top of the transformer tank to reach a vacuum of about 5 torr. The heaters in the oil filtration plant are switched on so that the oil temperature at any point will not go more than 60°C. Now the main tank oil is circulated through the oil filtration plant and also getting heated up. The insulation resistance of the winding is measured at an interval and the same is noted down against the oil temperature in the transformer tank. The inlet oil temperature as well as the outlet oil temperature from the filtration plant are also noted down at the regular interval. It is seen that initially when the temperature starts shooting up, the IR value starts coming down. This indicates that the moisture drops are getting distributed in the winding and the oil in form of vapour. After several circulation of oil through the machine, the insulation resistance becomes steady. On further continuation, the IR value starts rising. This indicates that the moisture is being expelled from the winding and oil. The graph will be as shown in the figure-3 given below: 1.4.1.5 Stopping Criterion: The completion of the drying of the transformer / reactor is decided based on the following results. 1. Steady increase of IR value of the windings. 15


2. BDV of the oil is more than 70 kV. 3. Moisture content of the oil is less than 10 PPM. 4. PI value is more than 2.

1.4.2

If it is established that the Transformer winding is wet and N2 purging is required, it is recommended to use the procedure given in section 1.4.3. The brief procedure for dry out of wet winding of Transformer/ Reactor by Vacuum pulling, N2 filling and heating is described in 1.4.3. However, for the detailed procedure, please refer to the Doc. No. D-2-03-XX-01-01: Maintenance Procedures for Switchyard Equipments-Part 1: EHV Transformers & Reactors

Figure-3 : Graph of readings taken during Drying out process of a Power Transformer

1.4.3

PROCEDURE FOR DRY OUT OF WET WINDING OF TRANSFORMER/ REACTOR BY VACUUM PULLING, N2 FILLING AND HEATING

1.4.3.1 Isolation Required: All the openings of transformer main tank like openings for coolers/radiators, conservator, OLTC etc. is to be properly isolated and totally blanked. 1.4.3.2 Procedure: a. Fill the main transformer/ reactor tank with Nitrogen (at least with the dew point of –50 °C or better) until it comes to a positive pressure of 0.15 Kg/cm2. It is kept at least 48 hrs. At the end of 48 hrs, dew point of N2 at outlet is measured. If the dew point is not within acceptable limits as per Table-I, dry out method should be continued. b. Heating from outside: While N2 circulation is in progress, the heaters are to be installed around the transformer tank as shown in the Annexure. The heaters are to be kept ON until we achieve a temperature of about 75–80 deg C of the core & winding of transformer.

16


c. Vacuum Pulling: After ascertaining that there is no leakage, pull out vacuum and keep the transformer/reactor under near absolute vacuum (1-5 torr) for about 96 hours running the vacuum pump continuously. The duration of vacuum can vary between 48 to 96 hrs. depending upon the dew point being achieved. Keep Vacuum machine ON and collect condensate for measurement. Observe the rate of condensate collection on hourly basis. Depending on the value of rate of condensate, continuation of further vacuum shall be decided. d. Nitrogen Circulation: Then the vacuum is broken with dry nitrogen. The dew point of nitrogen at the inlet is to be measured and should be of the order of - 50 °C or better. When the nitrogen comes to the positive pressure of 0.15 kg/cm2, it is stopped and kept for 24 hours. Heating from outside is to be continued while N2 circulation is in progress. Then the nitrogen pressure is released and the outlet nitrogen dew point is measured. If the dew point is within acceptable limits as per Table-I then the dryness of transformer is achieved. If not again the transformer is taken for vacuum treatment and then nitrogen is admitted as mentioned above and tested. The cycle to be continued till desired dew point as per Table-I is achieved. e. Periodicity: Periodicity of vacuum cycle may vary between 48-96 hrs. Initially two N2 cycles may be kept for 24 hrs. After that it may be kept for 48 hrs depending upon dew point being achieved. After completion of drying process, oil filling and hot oil circulation to be carried out before commissioning. Please ensure standing time as per table given below before charging. Transformer HV Rated Voltage (in kV) Up to 145kV 145 kV and up to 420kV Above 420 kV

Application of Vacuum & holding for (before oil filling ) (in Hours) 12 HRS 24 HRS

STANDING TIME After Oil circulation and before energising (in Hours) 12 HRS 48 HRS

36 HRS

120 HRS

After the expiry of this time, air release operation to be carried out in Buchholz relays and other release points given by the manufacturers before charging. For subsequent activities, proceed to format no. CF/ICT/06/ R-2 DATED 01/08/07 for ICT & format No. CF/SR/07/ R-2 DATED 01/08/07 for Reactor.

17


1.5 RELATION BETWEEN DIFFERENT UNITS (CONVERSION OF UNITS):

Pressure: 1 bar = 105 Pa = 750 Torr = 14.5 psi = 1.02kp/cm2 1 Torr = 1.33 mbar = 0.133 kPa 1 kPa(kilo-Pascal) = 103 Pa = 10 mbar = 7.501 Torr(mm of mercury) 1 MPa = 106 Pa 1 atmosphere=0.1 MPa=1.02kp/cm2= 14.5 psi Force: 1 kp = 9.807 N Weight: 1 ton=1000 kg=2200 lbs Temperature: 5 * (° F - 32) 9 9 F = * (° C )+ 32 5

°C= °

Volume: 1m3= 1000 litres=260 US gallons=220 Imp gallons 1 litre =0.26 US Gallons 1 US Gallons = 3.78 litres 1 litre =0.22 Imp Gallons 1 Imp Gallons Benchmarks: 1-mm mercury (Torr) is about 1 millibar or 0.1 kPa 1 m3 of oil weights 0.9 tons –say 1 ton 1000 US gallons of oil weights 3.5 tons

18


1.6 CHECK LIST FOR ENERGISATION OF TRANSFORMER/ REACTOR 1.6.1 PRELIMINARY CHECKS

1. Release air at the high points, like oil communicating bushings, buchholz petcock, tank cover and the cooling devices including headers, radiators, pumps, expansion joints etc. of the transformer 2. Check the whole assembly for tightness and rectify where necessary. 3. Check the general appearance and retouch the paint work if need be. 4. Check that the valves are in the correct position : • Tank : valves closed and blanked • Cooling circuit : valves open • Conservator connection : valves open • By-pass: valves open or closed as the case may be. • On-load tap changer : valves open 5. Check that the silica gel in the breather is blue and that there is oil in the breather cup ( oil seal ) 6. Check the oil level in the main conservator and the conservator of on-load tap changer, bushing caps, flanges, turrets, expansion bellows as per manufacturer’s recommendation. Level should correspond to 35º C mark on oil level gauges for BHEL transformers 7. Check the bushings : • Oil level ( bushings fitted with sight-glasses) • Adjustment of spark-gaps /arcing horn –gaps, if provided • Conformity of connection to the lines ( no tensile stress on the terminal heads) • Bushing CT secondary terminals must be shorted and earthed, if not in use. • Neutral bushing effectively earthed 8. Check the on-load tap changer : • Conformity of the positions between the tap changer control cubicle and the tap changer head • Adjustment of the tap-changer control cubicle coupling • Electric and mechanical limit switches and protective relays • Step by step operation- local and remote electrical operation as well as manual operation and parallel operation, if any • Signalling of positions 9. Check the quality of the oil: • Draw off a sample from the bottom of the tank • Carry out DGA and oil parameters test ( i.e. BDV, Moisture content, resistivity & tan δ at 90°C and IFT ) before energisation

19


10 Extraneous materials like tools, earthing rods, pieces of clothes, waste etc. should be removed before energisation 1.6.2 CHECKING OF AUXILIARY AND PROTECTIVE CIRCUITS

1. Check temperature indicator readings and their calibrations 2. Check the setting and working of the mercury switches of winding and oil temperature indicators and presence of oil in the thermometer pockets • Follow the same procedure for the thermal replicas 3. Check the operation of the buchholz relay and the surge protective relay of the tap-changer : • Alarm and tripping • Protections and signals interlocked with these relays 4. Check the insulation of the auxiliary circuits in relation to the ground by 2 kV megger for 1 min. 5. Check the earthing of the tank and auxiliaries like cooler banks at two places. 6. Measure the supply voltages of the auxiliary circuits 7. Check the cooling system : • Check the direction of rotation of the pumps and fans • Check the working of the oil flow indicators • Check the setting of the thermal overload relays • Go through the starting up sequences, control and adjust, if necessary, the relay time delays 8. Check that there is efficient protection on the electric circuit supplying the accessories and tightness of all electrical connections 9. Check the heating and lighting in the cubicles 10. Check the differential protection, over-current protection, restricted earth fault protection, over-fluxing protection etc. are in service and settings are as per CC/Engg recommendations. After the inspection / tests are completed, the transformer may be energised from the incoming side on NO LOAD. The initial magnetising current at the time of switching will be very high, depending upon the particular moment in the cycle. The transformer should always be soaked for few hours under constant care i.e. keep it energised for twelve hours. Excessive vibrations of radiator parts etc. should be located and corrected. The transformer hum should be observed for any abnormality. After that it may be checked for gas collection. Should the gas prove to be inflammable, try to detect the cause which may probably be an internal fault? If the breaker trips on differential /REF, buchholz or any other protective device, the cause must be investigated thoroughly before re-energizing the transformer/ reactor. After successful charging, performance of transformer / rector should be checked under loading; OTI/WTI readings should be monitored for 24 hours and ensured that they are as per loading. DGA samples may be sent as per Standard practice (after 24 hrs of energisation, one week, 15 days, one month and three months after charging, thereafter as per normal frequency of 6 months). Loading data may be forwarded to CC/OS and manufacturer (if requested by them).

20


2.0

PRE-COMMISSIONING CHECKS /TESTS FOR TRANSFORMERS & REACTORS Once oil filling is completed, various pre-commissioning checks/ tests are performed to ensure the healthiness of the Transformer/ Reactor prior to its energisation. Various electrical tests to be performed and their significance are given below.

Sr. Name of Test No. Check point

/ Purpose of test/ check

1.

Core insulation tests

To check the insulation between Core (CC&CL) and Ground

2.

Operational Checks Operational Checks on cooler bank (pumps & Fans), Breathers on protection System (silica gel or drycol), MOG, temperature gauges (WTI/OTI), gas actuated relays (Buchholz, PRD, SPR etc.) and simulation test of protection system

3.

Insulation Resistance measurement

4

Capacitance and Tan Measurement of C1 & C2 Capacitance and Tanδ in UST mode. δ measurement of Changes in the normal capacitance of an insulator indicate bushings abnormal conditions such as the presence of moisture, layer short -circuits or open circuits in the capacitance network.

5.

Capacitance and Tan Dissipation factor/Loss factor and capacitance measurement of δ measurement of winding is carried out to ascertain the general condition of the windings ground and inter-winding insulation

6.

Turns ratio (Voltage To determine the turns ratio of transformers ratio) measurement

7.

Vector Group Polarity

8.

Winding resistance To check for any abnormalities due to loose connections, broken measurement strands and high contact resistance in tap changers

9.

Magnetic test

10.

Floating Neutral This test is conducted to ascertain possibility of short circuit in a point measurement winding.

11.

Exciting/Magnetising To locate defect in magnetic core structure, shifting of windings, current measurement failures in turn to turn insulation or problems in tap changers. These conditions change the effective reluctance of the magnetic circuit thus affecting the current required to establish flux in the core

Test reveals the condition of insulation (i.e. degree of dryness of (IR) paper insulation), presence of any foreign contaminants in oil and also any gross defect inside the transformer (Failure to remove the temporary transportation bracket on the live portion of tapchanger part)

& To determine the phase relationship and polarity of transformers

Balance This test is conducted only in three phase transformers to check the imbalance in the magnetic circuit

21


12.

Vibration To measure the vibrations of core /coil assembly in the tank of measurement of Oil- the reactor. Movement of the core-coil assembly and shielding immersed Reactor structure caused by the time–varying magnetic forces results in vibration of the tank and ancilliary equipment. These vibrations have detrimental effects such as excessive stress on the core-coil assembly

13.

Operational on OLTCs

14.

Tests/ Checks on To ascertain the healthiness of bushing current transformer at the Bushing Current time of erection Transformers (BCTs)

15.

Frequency Response To assess the mechanical integrity of the transformer. Analysis (FRA) Transformers while experiencing severity of short circuit current measurement looses its mechanical property by way of deformation of the winding or core.

16

On-Line moisture-in- To estimate the moisture (water) content in paper insulation of oil measurement transformers through on-line moisture measurement kit (DOMINO / VAISALA)

17

Dissolved Gas Oil sample for DGA to be drawn from transformer main tank Analysis (DGA) of before commissioning and after 24 hrs. of charging. Subsequently oil sample

18.

Thermovision Infra- A thermo vision Camera determines the temperature distribution red scanning (IR on the surface of the tank as well as in the vicinity of the Jumper thermography) connection to the bushing. The information obtained is useful in predicting the temperature profile within the inner surface of tank and is likely to provide approximate details of heating mechanism. Thermovision scanning of transformer to be done at least after 24 hrs. of loading and repeated after one week.

2.1

checks To ensure smooth & trouble free operation of OLTC during operation.

CORE INSULATION TEST This test is recommended before the unit is placed in service after receipt at site or following modifications to the transformer that could affect the integrity of its core insulation and at other times, when indicated by DGA (key gases being ethane and/or ethylene and possibly methane) or usually during a major inspection. For core-insulation to ground test, remove the cover of the terminal block, Disconnect the closing link that connects the two terminals CL-G. Apply 1.0 kV direct voltage between CL and CC + G (core grounding strap). The tank shall be grounded during the test. The insulation value after 1min. test-time should be minimum 10 MΩ for new transformer at the time of commissioning and minimum 1000kΩ for service aged transformers. Core insulation resistance is generally more than 100 MΩ for new assembled transformer when tested at factory. For Shunt Reactors, the measurement to be done by 3.5kV (DC) megger, Min value should be 1000kΩ.

22


2.2

• • • • •

• • • • 2.3

OPERATIONAL CHECKS ON PROTECTION SYSTEM Operational Checks on Breathers (Conventional Silcagel or Drycol as supplied with the transformers) Visual check of MOG of Main Conservator Marshalling Box & Kiosk Checks Valve Operational Checks Checks on Cooling System Checks on cooling fans-rotation, speed & Control (Manual /temp /load) setting checks Checks on Cooling pumps- rotation, vibration/noise, oil flow direction Checks on temperature Gauges (OTI/WTI-Calibration and Cooler Control , alarm & trip setting tests Checks on gas actuated (SPRs/ PRDs/ Buchholz) relays –Operational checks by simulation as well as shorting the respective contacts as applicable Checks on tightness of Terminal connectors - micro-ohm measurement of each connection Checks on Transformer protection (differential, SEF, Over-current & transformer stability tests etc.) INSULATION RESISTANCE (IR) MEASUREMENT

IR measurements shall be taken between the windings collectively (i.e. with all the windings being connected together) and the earthed tank (earth) and between each winding and the tank, the rest of the windings being earthed. Before taking measurements the neutral should be disconnected from earth. Following table gives combinations of IR measurements for auto-transformer, three -winding transformer & Shunt Reactor. For Auto-transformer HV + IV to LV HV + IV to E LV to E

For 3 winding transformer HV + IV to LV HV + LV to IV HV + IV +LV to E

For

Shunt Reactor HV to E

Where HV-High voltage, IV-Intermediate voltage, LV-Low voltage/Tertiary voltage windings, E- Earth

Insulation resistance varies inversely with temperature and is generally corrected to a standard temperature (usually 20 °C) using table (Source: BHEL instruction Manual) as given below. Difference in temperatures ( °C) 10 20 30 40 50

Correction Factor (k) 1.65 2.6 4.2 6.6 10.5

Unless otherwise recommended by the manufacturer the following IR values as a thumb rule may be considered as the minimum satisfactory values at 30°C (one minute measurements) at the time of commissioning. 23


Rated Voltage class of Minimum desired IR value at 1 minute winding (Meg ohm) 11kV 300 MΩ 33kV 400 MΩ 66kV & above 500 MΩ The ratio of 60 second insulation resistance to 15 second insulation resistance value is called dielectric absorption coefficient or Index (DAI). For oil filled transformers with class A insulation, in reasonably dried condition the absorption coefficient at 30°C will be more than 1.3. Polarization Index Test is ratiometric test, insensitive to temperature variation and may be used to predict insulation system performance. The polarization index test is performed generally by taking mega ohm readings at the following intervals at a constant dc voltage: 1 min and then every minute up to 10 min. The polarization index is the ratio of the 10 min to the 1 min mega ohm readings. PI= R10 / R1 (dimensionless), Where PI is Polarisation Index and R is resistance The following are guidelines for evaluating transformer insulation using polarization index values: Polarization Index Less than 1 1.0-1.1 1.1-1.25 1.25-2.0 2.0 – 4.0 Above 4.0

Insulation Condition Dangerous Poor Questionable Fair Good Excellent

A PI of more than 1.25 and DAI of more than 1.3 are generally considered satisfactory for a transformer when the results of other low voltage tests are found in order. PI less than 1 calls for immediate corrective action. For bushings, an IR value of above 10,000 MΩ is considered satisfactory. 2.4

CAPACITANCE AND TAN δ MEASUREMENT OF BUSHINGS C& Tan δ measurement of bushings shall be done at 10kV with fully automatic test kit so as to have reliable test result. •

For 3-Ph auto-transformer, short together all 400kV, 220kV and Neutral (isolated from earth) Bushings. Also short all 33kV Bushings and earth the same. Measurement of C1 Capacitance and Tanδ δ: Connect the crocodile clip of the HV cable to the top terminal of the shorted HV/IV bushings. Unscrew the test tap cover, Insert a pin in the hole of the central test tap stud by pressing the surrounding contact plug in case of 245 kV OIP Bushing and remove the earthing strip from the flange by unscrewing the screw (holding earth strip to the flange body) in case of 420 kV OIP Bushing. Connect the LV cable to the test tap (strip/central stud) of the bushing under test to the C & TAN δ KIT through a screened cable and earth the flange body. Repeat the test for all Bushings by changing only LV lead connection of the kit to test tap of the Bushing which is to be tested 24


• •

•

•

2.5

Measurement of C2 Capacitance and Tanδ δ : HV lead to be connected to the test tap of the bushing under test (if required additional crocodile type clip may be used) and LV of the kit to be connected to the ground. HV of the bushing is to be connected to the Guard terminal of the test kit. Test to be carried out in GSTg mode at 1.0kV. For measurement of 33kV Bushing Tan Delta, earth HV/IV Bushings (already shorted). Apply HV lead of the Test kit to shorted 33kV Bushings and connect LV lead of the test kit to Test tap of the Bushing under test. Measurements shall be made at similar conditions as that of a previous measurement. The oil-paper insulation combination of bushings exhibit fairly constant tan delta over a wide range of operating temperature. Hence, effort is to be made for testing at temperature near to previous test and Correction factor need not be applied. Do not test a bushing (new or spare) while it is in its wood shipping crate, or while it is lying on wood. Wood is not as good an insulator as porcelain and will cause the readings to be inaccurate. Keep the test results as a baseline record to compare with future tests. It is to be ensured that Test Tap points are earthed immediately after carrying out the measurements for that particular Bushing and earthing of test tap to be ensured by carrying out continuity test. CAPACITANCE AND TAN δ MEASUREMENT OF WINDINGS The combination for C & tanδ measurement of winding is same as that of measurement of IR value. The summery of probable combination is given below:

AutoTransformer (Two winding) HV HV + IV to LV HV HV + IV to E TV LV to E

Test Mode

Shunt TestFo 3 winding Reactor Mode Transformer

USTHV HV to E GSTg GSTg

GST

HV to LV1 HV to LV2 LV1 to LV2 HV to Ground LV1 to Ground LV2 to Ground

Test Mode UST UST UST GSTg GSTg GSTg

Table 2: Combination for C & tanδ measurement of winding for various transformers/ Shunt Reactor

•

•

Ensure that test specimen is isolated from other equipments. Removal of Jumpers from Bushings is Pre-Requisite for C & Tan δ Measurement of Windings. For ICTs (Auto-Transformers): Shorting of all three phase Bushings (400kV&220kV) and neutral to be done. In case of single phase, 400kV, 220kV and neutral Bushings to be shorted Capacitance and Tan δ measurement of windings should be done in following combinations: Test No. 1.

Winding Combination HV-IV/LV

Test mode UST

Cap Symbol CHL

Test lead Connection

Remarks

HV lead of test kit to HV/IV bushings of transformer LV lead of test kit to LV

25


2. 3. 4

5 6.

•

•

2.6

HV-IV/ LV+G HV-IV / LV with Guard HV-IV/LV

GST GSTg

CHL +C HG C HG

UST

CHL

bushing of transformer -do- do-

LV to Guarded

be

LV lead of test kit to HV/IV bushings of transformer HV lead of test kit to LV bushing of transformer -do-

LV/ HV-IV GST CHL +C LG +G LV/ HV-IV GSTg C LG -doHV to with Guard Guarded Table 3:.Winding combination for C & tan δ measurement for auto transformer

be

Measurement inter-check can be done by calculating C1= C2-C3 & C4= C5- C6 & DF1=C2DF2-C3DF3 / C2-C3= C4DF4-C5DF5 / C4-C5 Where C stands for capacitance and DF for dissipation factor or tan δ and attached suffix (1…6) denotes the sr. no. of test in above table. For Reactors: All 400kV and neutral Bushings to be shorted. HV of the test kit to be connected to shorted Bushings and LV of the test kit to be connected to Earth connection. Measure the Capacitance and tan Delta in GST mode. Neutral connection with earth/ NGR to be isolated before the test. TURNS RATIO (VOLTAGE RATIO) MEASUREMENT Ratio measurements must be made on all taps to confirm the proper alignment and operation of the tap changers. The test should preferably be performed by Transformer turns ratio (TTR) meter. They have a hand crank power supply, with the voltages commonly used being very low, such as 8-10 V and 50-60 Hz, so that the test may be performed on a transformer even when the oil is removed. Two windings on one phase of a transformer are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector, with exciting current also being measured in most cases. Results of the transformation turns or voltage ratio are absolute, and may be compared with the specified values measured during factory testing. The turns ratio tolerance should be within 0.5 % of the nameplate specifications. For three phase Y connected winding this tolerance applies to phase to neutral voltage. If the phase-to-neutral voltage is not explicitly indicated in the nameplate, then the rated phase-to-neutral voltage should be calculated by dividing the phase-to-phase voltage by √3. Open turns in the excited winding will be indicated very low exciting current and no output voltage. Open turns in the output winding will be indicated by normal levels of exciting current, but no or very low levels of unstable output voltage. The turns ratio test also detects high-resistance connections in the lead circuitry or high contact resistance in tap changers by higher excitation current and a difficulty in balancing the bridge 26


2.7

VECTOR GROUP & POLARITY The test shall be conducted with three phase supply (415V, 3 phase supply) and voltmeters. By the measured voltage data it should ensure that the desired conditions of vector group and polarity are fulfilled

Example for D Y1

CONNECT 1U to 2U MEASURE 1W-2V, 1W-2W, 1U-2W, 1V-2V, 1V-2W VOLTAGE RELATION

1U

1W-2V= 1W-2W

2U

1W-2V< 1U-1W

1V 2W

2V

1V-2V<1V-2W

1W Dy-I

1V-2V <1U-1W

Fig 4 : For HV-Delta / LV-Star Transformer Example for Y D11

CONNECT 1U to 2U 1U+2U

MEASURE 1W-2V, 1W-2W, 1U-2W, 1V2V, 1V-2W

2V 2W

VOLTAGE RELATION 1W-2W = 1V-2W

N

1W-2U > 1V-2U 1U-N=1U-2W+2W-N

1W

1V

Yd11 Fig 5: For HV-Star / LV-Delta Transformer Please Note : Most of the auto transformers in POWERGRID are of YD11configuration. 2.8

WINDING RESISTANCE MEASUREMENT As the transformer resistance is Low resistance, the measurement has to be carried out with the help of Kelvin Double Bridge / Transformer ohm meter. Normally winding resistance values 1 ohm or above is measured using Wheatstone Bridge and winding resistance values less than 1 ohm is measured using micro-ohm meter or Kelvin Bridge. To reduce the high inductive effect it is advisable to use a sufficiently high current to saturate the core. This will reduce the time required to get a stabilized reading. It is essential that temperatures of the windings are accurately measured. Care shall be taken that self inductive effects are minimized. 27


Care also must be taken to ensure that direct current circulating in the windings has settled down before the measurement is done. In some cases this may take several minutes depending upon the winding inductance. The winding resistance shall be preferably done when the difference in the top and bottom temperature of the winding (temperature of oil in steady-state condition) is equal to or less than 5°C. The winding resistance should preferable be carried out last after completion of all other LV tests, as after this test core gets saturated and tests like magnetizing current, magnetic balance etc. carried out after winding test may be affected and indicate a misleading results, if the core is not de-magnetized before carrying out these tests. For star connected auto-transformers the resistance of the HV side is measured between HV terminal and IV terminal, then between IV terminal and the neutral AT ALL TAPS. The tap changer should be changed from contact to contact so that contact resistance can also be checked. For delta connected windings, such as tertiary winding of auto-transformers, measurement shall be done between pairs of line terminals and resistance per winding shall be calculated as per the following formula: Resistance per winding = 1.5 x Measured value Take the winding temperature reading while doing the resistance measurement. Calculate the resistance at 75°C as per the following formula R75 = Rt (235+75)/(235+t ), 2.9

Where Rt = Resistance measured at winding temperature t

MAGNETIC BALANCE TEST This test is a low voltage test conducted at factory and site by applying single phase voltage between phase and neutral of a winding and measuring voltage induced in other two phases of the same winding. Keep the tap in nominal tap position. Disconnect transformer neutral from ground. Apply single phase 230 V across one phase of Highest Voltage (HV) winding terminal and neutral (call it v1) then measure voltage in other two HV terminals across neutral (call them v2 and v3 respectively). Repeat the test for each of the three phases. Repeat the above test for Intermediate Voltage (IV) winding also The identical results confirm no damage due to transportation. The following points may be noted. Transformer neutral should be disconnected from ground (i) No winding terminal should be grounded, otherwise results would be erratic and confusing. 28


(ii) (iii)

Zero voltage or very negligible voltage induced in any of the other two phases shall be investigated. It is proposed that a set of readings should be taken for information and comparison later during service of the transformer.

Also the applied voltage may be expressed as 100% voltage and the induced voltage may be expressed as percentage of the applied voltage. This will help in comparison of the two results when the applied voltages are different. The voltage induced in the centre phase shall be 50 to 90% of the applied voltage. However, when the centre phase is excited then the voltage induced in the outer phases shall be 30 to 70% of the applied voltage. 2.10

FLOATING NEUTRAL POINT MEASUREMENT

This test is also a 3 phase 415 volt where the supply is applied across HV windings or IV winding as the case may be after disconnecting the transformer neutral from the ground. For a healthy transformer, when 3 phase balance voltage is applied, the voltage between neutral and ground is zero or otherwise a negligible voltage will appear. But in case there is a short circuited winding, the voltage between the neutral and the ground is appreciable. This test will also help in detecting the gradual deterioration or development of fault in the winding during service. 2.11

EXCITING/ MAGNETISING CURRENT MEASUREMENT

This test should be done before DC measurements of winding resistance to reduce the effect of residual magnetism. Magnetising current readings may be effected by residual magnetism in the core. Therefore, transformer under test may be demagnetised before commencement of magnetising current test. Three-phase transformers are tested by applying Single-phase 10 kV voltage to one phase (HV terminals) and keeping other winding open circuited and measuring the current at normal, minimum and max. tap positions. Keep the tap position in normal position and keep HV and LV terminals open. Apply 1phase 10kV supply on IV terminals. Measure phase to phase voltage between the IV terminals and current on each of the IV terminals. The set of reading for current measurement in each of the tap position should be equal. Unequal currents shall indicate possible short circuits in winding. Results between similar single-phase units should not vary more than 10 % .The test values on the outside legs should be within 15 % of each other, and values for the centre leg should not be more than either outside for a three-phase transformers. Results compared to previous tests made under the same conditions should not vary more than 25%. If the measured exciting current value is 50 times higher than the value measured during pre-commissioning checks, then there is likelihood of a fault in the winding which needs further analysis. The identical results confirm no damage due to transportation. The availability of test data of normal condition and faulty condition results help us to analyze the problem in future.

29


2.12

VIBRATION MEASUREMENT OF OIL-IMMERSED REACTOR

The shunt reactor under test shall be completely assembled in normal operating condition with cooling equipments, gauges and accessories. The shunt reactor shall be energised at rated voltage and frequency. Three phase excitation for 3-ph units. The shunt reactor should be mounted on a level surface that will provide proper bearing for the base, in order to eliminate the generation of abnormal tank stresses The vibration of shunt reactor shall be measured by transducers, optical detectors or equivalent measuring devices. The measuring equipment should be accurate within +/- 10 % at 2nd harmonic of the exciting frequency. The peak-to-peak amplitude shall be determined by direct measurement or calculated from acceleration or velocity measurement 2.13

OPERATIONAL CHECKS ON OLTC

Following operational checks should be carried out during pre-commissioning: • • • • • • • • • • • •

Visual Inspection Of Equipment Manual Operation On All Taps ( Local ) Over Load Device Of Driving Motor Local Operation (Electrical) Remote Operation (Electrical) Tap Position Indicator Check Operation With Master Follower Scheme ( Parallel Operation ) Out Of Step Relay Step By Step Contactor Limit Switch Winding Resistance At All Taps

The following additional check points /guide lines for OLTC is recommended in consultation with OLTC manufacturer to ensure the absence of problems and proper operation: a. Function of control switches b. OLTC stopping on position c. Fastener tightness d. Signs of moisture such as rusting, oxidation or free standing water and leakages e. Mechanical clearances as specified by manufacturer’s instruction booklet f. Operation and condition of tap selector, changeover selector and arcing transfer switches g. Drive mechanism operation e. Counter operation i. Position indicator operation and its co-ordination with mechanism and tap selector positions j. Limit switch operation k. Mechanical block integrity l. Proper operation of hand-crank and its interlock switch m. Physical condition of tap selector n. Freedom of movement of external shaft assembly o. Extent of arc erosion on stationary and movable arcing contacts 30


p. Inspect barrier board for tracking and cracking q. After filling with oil, manually crank throughout entire range r. Oil BDV and Moisture content (PPM) to be measured and recorded (Min BDV should be 50 kV and Moisture content should be less than 20 PPM) 2.14

TESTS/ CHECKS ON BUSHING CURRENT TRANSFORMERS (BCTS)-

Continuity, Polarity and secondary winding resistance tests of individual cores of Bushing CTs 2.15

FREQUENCY RESPONSE ANALYSIS (FRA) MEASUREMENT

Frequency Response Analysis (FRA) is made to assess the mechanical integrity of the transformer which may get disturbed due to transportation socks or S.C. forces experienced in service. FRA signatures will be taken after assembly and oil filling and compared with factory testing to ensure the healthiness of core /coil assembly during transportation. These signatures will be benchmark for future reference. The FRA signatures should be analysed in conjunction with Impact Recorder readings. Report of Impact recorder readings is to be obtained from manufacturer. 2.16

ON-LINE MOISTURE-IN-OIL MEASUREMENT

Once the transformer/ reactor is charged and loaded, on-line moisture –in-oil measurement can be made to assess the moisture present in the winding insulation for future ref. 2.17

DISSOLVED GAS ANALYSIS (DGA) OF OIL SAMPLE

Dissolved Gas Analysis (DGA) is a powerful diagnostic tool to detect any incipient fault developing inside the oil-filled equipment. The oil sample is to be taken after oil filling (before commissioning ) as a benchmark and there after 24hrs of charging, 7 days, 15 days, one month and three months after charging to monitor the gas build up if any. The oil samples are to be sent to CIOTL/IOTL/STL for DGA and first two samples for oil parameter testing also. 2.18

THERMOVISION SCANNING (IR THERMOGRAPHY)

Once the transformer/ reactor is charged and loaded, Thermovision scanning is to be carried out to see any hotspots. For detailed procedure for each test, please refer Transformer and Reactor Maintenance manual (Doc No. D-2-03-XX-01-01)-First Revision, Part B, C & D.

31


3.0

PRE-COMMISSIONING CHECKS /TESTS FOR OTHER SWITCHYARD EQUIPMENTS

Once erection is completed, various pre-commissioning checks/ tests are performed to ensure the healthiness of the switchyard equipments prior to their energisation. Various major electrical tests to be performed and their significance are given below. Sr. No.

Name of Test / Check Purpose of test/ check point

3.1

Tan δ & Capacitance measurement of CT, each stack of CVT & total capacitance, CB voltage grading capacitor & each stack of Surge Arresters

3.2

Checks/ Tests applicable for CTs

3.2.1

Polarity test for CT

To ascertain whether the polarity markings are correct or not as per drawing.

3.2.2

Magnetization characteristics of CT

To prove that the turns of CTs secondary windings are not short circuited and to check healthiness of CT cores. The magnetizing currents at KPV (Knee point voltage) shall be less than the specified value. The ratio of secondary and primary voltage shall also be measured.

3.2.3

Ratio test for CT

The ratio errors of the primary to the secondary currents should within specified ratio errors.

3.2.4

IR measurement of CT Changes in the normal IR value of CT indicate abnormal conditions such as presence of moisture, dirt, dust, crack in (Primary & Secondary insulator of CT and degradation of insulation. windings)

3.2.5

DGA test of CT oil

3.3 3.3.1

Checks/ Tests applicable for Circuit Breakers

3.3.2

Measurement of Circuit Breaker Operating Timings including PIR Timings

The purpose of the dissipation factor measurement of high voltage insulation is to detect incipient weaknesses in HV insulation. The most important benefit to be gained from this measurement is to obtain a “benchmark reference reading” on costly and high voltage equipment when the equipment is new and insulation is clean, dry and free from impurities. Tan delta & Capacitance values shall be comparable with factory test results and in no case shall be more than 0.5 %.

This test shall be conducted after 30 days of commissioning. The purpose is to identify evolving faults in the CT and DGA values shall be comparable with factory values (if available)

Dew point measurement Dew point of SF6 gas is to measure moisture content in of SF6 gas SF6 gas which shall indicate whether CB evacuation is done properly or not. This test shall be carried out preferably at rated pressure of SF6 gas. To measure closing/ tripping/ CO timings. These timings should be within permissible limits and shall be comparable with factory values. Pole discrepancies and Break to Break discrepancies shall be less than specified values. 32


3.3.3

DCRM Contact Travel Measurement / DC injected currents and trip/ close coil currents.

3.3.4

Operational lockout To ensure various lockout operation of CB by simulating checking for EHV the actual conditions at the specified pressure of oil/ air/ Circuit Breakers operating medium.

3.3.5

Measurement of static This test is conducted to evaluate healthiness of Main contact resistance contacts. 100 Amp DC is injected and voltage drop is measured across each CB contact to compute contact resistance.

3.3.6

Checking the Pumping feature

3.3.7

Checking the Anti- To check correct operation of Thermostat provided for anti Condensation Heaters condensation heaters.

3.3.8

Pole discrepancy relay To test tripping of CB in case of pole discrepancy more testing than 2.5 seconds or specified value.

3.3.9

Checking the priming pressure

3.4

Checks/ Tests applicable for CVTs

3.4.1

CVT polarity, Ratio test

3.5

Checks/ Tests applicable for Isolators

3.5.1

MILLIVOLT Drop test

The voltage drop gives a measure of resistance of current carrying part and contacts by injecting minimum 100 A DC current.

3.5.2

50 operation tests

To test operation of contacts etc with jumpers connected and contact resistance to be measured after 50 operations. There shall not be any change from the previous value.

3.6

Checks/ Tests applicable for Surge Arrestors

3.6.1

Third Resistive (THRC) arrestors

3.7

Checks/ Tests for other areas/ equipments

DCRM is the technique for measuring Contact Resistance during operation (Close/ Trip) of a circuit breaker with a delay Tco of 300ms. A DC current of at least 100 Amp is injected through the circuit breaker. The current and voltage drop are measured and resistance is calculated. The resistance and travel versus time data provides useful information on the condition of the circuit breaker contacts and is used as a diagnostic tool. DCRM test signatures shall be approved by Corporate OS.

Anti- By giving simultaneous close/ trip commands, CB hunting shall not take place by operation of Mechanical/ Electrical anti pumping feature.

N2 This test is to check healthiness of N2 accumulators provided in Hydraulic drive mechanisms. N2 priming pressure shall be as per the rated pressure. This test is conducted in the same manner as for CT to determine correct CVT polarity, ratio and phasor group.

Harmonic To monitor healthiness of Surge arrestors by monitoring Current third harmonic resistive current from the leakage current. for surge This test is to be conducted after charging of Las. The value of THRC shall be less than 30 µA.

33


3.7.1

Earth resistance To ensure value of earth resistance is below 1 ohm. measurement

3.7.2

Primary injection test

current To test the whole protection system & detect any incorrect CT polarity, setting of relay in wrong sequence etc.

3.7.3

Secondary injection test

current Conducted for testing of protecting devices, circuit breakers, trip coils, motor overloads etc.

3.2 TAN DELTA & CAPACITANCE MEASUREMENT OF CT, CVT, CB VOLTAGE GRADING CAPACITORS AND LA STACKS To measure dissipation factor/loss factor (Tan delta) and Capacitance measurement of EHV class CTs, CVTs, CB Voltage Grading Capacitors & LA stacks by applying test voltages upto 10kV. Dissipation Factor Dissipation factor/loss factor (Tan delta)is defined as the ratio of resistive component(Ir) of current to that of capacitive current (Ic) flowing in an insulating material. Power Factor Power factor is the ratio of resistive current to that of total current. For very low value of resistive currents, values of dissipation factor and power factor are same (upto 2%). UST Test set connected for Ungrounded Specimen Test mode. This is used when specimen is isolated from earth e.g. Transformer bushing, CTs with test tap, CVTs and CB voltage grading capacitors. The test mode is often used to reduce the effect of stray capacitance losses to ground, and to reduce the effect of interference pickup from energized apparatus. GST Test set connected for grounded specimen test mode. This is used when specimen do not have two specific points (isolated from ground) for Tan delta measurement e.g. Transformer/Reactor winding, CTs without test tap etc. GSTg This test is used to separate the total values of a GST test into separate parts for better analysis. Often this test is used with GST test to confirm the test readings made using the UST mode. TEST EQUIPMENT 10 kV Capacitance and Tan Delta test set having normal and reverse mode of operation as well as Interference Suppression Units. PRECAUTIONS a) There should be no joints in testing cables. b) HV lead should be double shielded / screened. Both the shields should not get shorted otherwise tests in GST/GSTg modes, shall not be possible. Check the same by Insulation Tester(100V) c) Test leads should not touch any live part. d) Never connect the test set to energised equipment e) The ground cable must be connected first and removed at last f) High voltage plugs should be free from moisture during installation and operation. 34


g) It should be ensured that whole testing equipment alongwith Operation Manual of the kit testing procedures are available at testing site. Testing must be carried out in presence of testing personnel only. h) After testing with high voltage (10 kV), test terminals must be grounded before being touched by any personnel. TESTING PROCEDURE: Typical arrangement for Tan δ measurement is given below:

Fig. 6 Typical Sketch of Tan δ Measuring kit A)

CURRENT TRANSFORMERS

CTs with test taps: 1. Tan delta tap should be disconnected from ground. 2. High voltage lead from tan delta kit should be applied to primary(HV) Terminal and LV lead should be connected to the Tan delta test tap. 3. Before applying HV, interference is to be nullified using Interference suppression unit (ISU). 4. Measurements have to be taken in UST – Mode. 5. Standard procedure(as specified by kit supplier) for measuring capacitance and tan delta in charged switchyard/induced voltage conditions should be followed for measurement of capacitance and tan delta values. 6. Measurement to be carried out at 2kV and then at 10kV. 7. Carry out the measurements in main and reverse mode/polarity and then compute the average value.

B)

CB VOLTAGE GRADING CAPACITOR

1. Connect LV cable to the middle of the double interrupter. 2. Connect HV cable to the other end of the Grading capacitor to be tested. 3. The opposite end of the grading capacitor has to be grounded using earth switch. 35


4. Before applying HV, interference is to be nullified using Interference suppression unit (ISU). 5. Measurements have to be taken in UST Mode only 6. Disconnect the HV cable and connect the same to the other grading capacitor and ground the previous grading capacitor. Now the second grading capacitor is ready for testing. 7. Standard procedure (as specified by kit supplier) for measuring capacitance and tan delta in charged switchyard/induced voltage conditions should be followed 1. Measurements are to be carried out at 10 kV. 2. Carry out the measurements in main and reverse mode/polarity and then compute the average value. C)

CAPACITOR VOLTAGE TRANSFORMES

1. Testing procedure for Top and Middle Stacks: (a) Apply 10 KV between flanges of Top/Middle stacks (whichever is being tested) (b) Carry out measurements in UST mode at 10.0 KV (c) While measuring Middle/ Bottom stacks, Top/ middle stacks to be shorted. 2. Testing procedure for Bottom Stack connected to EMU PT. (a)

(b) (c) (d) (e)

Connect HV of the test kit at the top flange of bottom stack and LV of the test kit to the EMU Tank/ Ground. HF point to be grounded. Earth connection of the neutral of the PT to be opened/ isolated from ground. Top of CVT to be guarded. Carry out measurements in GSTg mode at 10.0 KV Repeat the Test with neutral of PT connected to ground. In case Tan delta value is negative or erratic, only capacitance values are to be monitored.

3. Standard procedure (as specified by kit supplier) for measuring capacitance and tan delta in charged switchyard/ induced voltage conditions should be followed. 4. Carry out the measurements in main and reverse mode/ polarity and then compute the average value. D)

SURGE ARRESTORS

1. Testing procedure for Top, Middle and Bottom Stacks: (a) Apply 10 KV between flanges of Top/Middle/ Bottom stacks (whichever is being tested) (b) Carry out measurements in UST mode at 10.0 KV (c)

While measuring Middle/ Bottom stacks, Top/ middle stacks to be shorted.

2. Standard procedure (as specified by kit supplier) for measuring capacitance and tan delta in charged switchyard/ induced voltage conditions should be followed. 3. Carry out the measurements in main and reverse mode/ polarity and then compute the average value. 36


3.2 CHECKS/ TESTS APPLICABLE FOR CTs 3.2.1

POLARITY TEST FOR CT

A centre zero voltmeter is connected across CT secondary. A 1.5 Volt battery is touched to primary of CT secondary. The deflection of pointer should be similar in case of each CT core. At any instant current entering the primary from P1 the current should leave secondary from the terminal marked S1. A setup shown in the Figure 7 can show whether the polarity markings are correct or not. When the key is pressed, current enters the primary through terminal P1, the voltmeter connected as shown, should read positive. A general arrangement of polarity test setup is indicated in Fig. 8.

P2

S2

R -

KEY

-

B

Y P1

P1

V

1.5 V

+

P1

S1

Figure – 7

+

S1

+

3.2.2

P1

+ -

S2

V -

Figure - 8

MAGNETIZATION CHARACTERISTICS OF CTs PRECAUTIONS a) There should be no joints in testing leads/cables. b) It should be ensured that whole testing equipment along with testing procedures are available at testing site. Testing must be carried out in presence of testing personnel only. Test Equipment: Voltage source of 5 kV, Voltmeter of range 0 to 5 kV, Ammeter, of range 0 to 500 Amps, testing leads/cables etc. Test Procedure: Make connections as per diagram shown below (Fig- 9). After making proper connections, applied voltage is increased from zero to rated Knee Point Voltage in steps of 25%, 50%, 75% and 100%. Measure the current drawn by the CT secondary core at respective applied voltages and record the test results

37


Figure - 9

Knee Point Voltage is normally defined as the voltage at which 10% increase in the applied voltage causes 30 to 50% increase in secondary current. The magnetization current at rated Knee Point Voltage should not be more than the specified/designed value. A curve can be drawn between applied voltage and magnetizing current. Typically, the curve drawn should be like the one given below:-

600 10 %

VOLTAGE

500 400

300 30 % 200

100

MAGNETIZING CURRENT

TYPICAL MAGNETIZATION CURVE

Figure - 10 From the curve it can be implied that up to rated KPV (Knee Point Voltage), the VI curve should be almost a straight line. However, if this line is not linear, this indicates that the magnetizing characteristics are not desirable. If the slop of the curve starts increasing, it indicates that magnetizing induction becomes low and total primary current is utilized in exciting the core alone. Consequently, out put of CT secondary disappears.

38


3.2.3

RATIO TEST FOR CURRENT TRANSFORMER The ratio check has to be carried out as indicated in Fig-11 below. PRIMARY CURRENT INJECTION SET

A1

A2 A1 : PRIMARY CURRENT A2 : SECONDARY CURRENT

Figure - 11 It is customary to conduct this in conjunction with the primary injection test. Current is passed through the primary circuit with the secondary winding circuit to its normal circuit load. The ratio of the primary to the secondary currents should approximate closely to that stamped under CT identification plate. Alternatively, ratio test is to be conducted as per the following method (Fig-12).

Figure - 12 Apply voltage from secondary of the CT and measure voltage in primary winding. Increase voltage in secondary up to rated KPV/ ISF and by recording Primary Voltage, compute ratio of V1/ V2. The ratio should match with the specified value. 3.2.4

INSULATION RESISTANCE TRANSFORMER

MEASUREMENT

OF

CURRENT

PRECAUTIONS a) There should be no joints in testing cables. b) Test leads should not touch any live part. c) Megger body should be earthed (if separate terminal is provided). d) Surface/terminals should be cleaned. e) IR measurement should be carried out preferably in dry and sunny weather. f) Never connect the test set to energised equipment. g) The ground terminal must be connected first and removed at last. 39


h) High voltage plugs should be free from moisture during installation and operation. i) If oil traces are found on the surface of CT, the same should be cleaned by Methyl Alcohol only. Petrol or diesel should never be used. j) It should be ensured that whole testing equipment along with testing procedures are available at testing site. Testing must be carried out in presence of testing engineer only. k) After testing with high voltage, test terminals must be grounded before being touched by any personnel. l) Test leads should be properly screened/ shielded. Connect the Megger as per figure given below. Connect the HV terminal to the Primary terminal of CT by using crocodile clip for firm grip.

Fig. 13 Typical Arrangement for IR measurement Carry out the measurement as per standard procedure given by the kit supplier. A test voltage as specified is applied as per the above connections and successive readings are taken. Values of IR should be recorded after 15 seconds, 60 seconds and 600 seconds. Ambient temperature and weather conditions are to be recorded. 3.2.5

DGA Test of CT Oil: Oil samples to be collected in 300ml bottles and to be sent to CIOTL Hyderabad for testing. Test results should be comparable to factory values. In case of any deviation, test results may be forwarded to CC-OS for approval.

3.3 CHECKS/ TESTS APPLICABLE FOR CIRCUIT BREAKERS 3.3.1 DEW POINT MEASUREMENT OF SF6 GAS FOR CIRCUIT BREAKER Dew Point is the temperature at which moisture content in SF6 gas starts condensing. Dew Point at rated pressure of CB: Dew Point when measured keeping regulating valve in service at the outlet of dew point kit to allow required flow rate of gas, is called at rated pressure of CB. Inlet valve is opened completely.

40


Dew Point at atmospheric pressure : Dew Point when measured by regulating the gas flow at the inlet of dew point kit and keeping outlet regulating valve ( if provided) in fully open condition so that flow rate of gas is maintained as required, is called at atmospheric pressure. TESTING PROCEDURE a) Make the connections to the kit from CB pole ensuring that regulating valve is fully closed

at the time of connections of the Dew Point kit. b) By regulating the flow rate of SF6 gas (0.2 liter/min to 0.5 liter/min - ref. IEC 60480), the

value of dew point is observed till it becomes stable. c) If the regulating valve is provided at outlet of the dew point kit then values as given in

Doc. no. for rated pressures are to be monitored. Dew Point of SF6 gas varies with pressure at which measurement is being carried out. This is due to the fact that Saturation Vapour Pressure decreases with increase in Pressure of the SF6 gas. Hence, dew point of SF6 gas at higher pressure is lower than dew point at atmospheric pressure Therefore, it is to be ensured that if measurement has been done at a pressure other than the atmospheric pressure, same is to be converted to the atmospheric pressure as given in the table below used at the time of commissioning for various CB manufacturers: Method for converting dew point at different gas pressures, is given/described in IEC-60480. Sl. No. Make of Dew point at Dew point at CB rated pressure Atmospheric Pressure ( Limit ) 1 BHEL (-) 15° C - 36° C 2 M&G - 39° C 3 CGL (-) 15° C - 35° C 4 ABB (-) 15° C - 35° C 5

NGEF

(-) 15° C

- 36° C

3.3.2 MEASUREMENT OF CIRCUIT BREAKER OPERATING TIMINGS INCLUDING PRE INSERTION RESISTOR TIMINGS PRECAUTIONS a) There should not be any joint in testing cables. b) Test leads should not touch any live part. c) Never connect the test set to energised equipment. d) The ground cable must be connected first and removed at last. e) High voltage plugs should be free from moisture during installation and operation. f) Circuit Breaker Analyser body should be earthed ( if separate earth is provided). g) It should be ensured that whole testing equipment alongwith testing procedures are available at testing site. Testing must be carried out in presence of testing personnel only. h) Surface/terminals should be cleaned where the connections for testing are to be made. i) Clean earth point with sand paper/wire brush where earth terminal is to be provided. j) Ensure that all the poles trip simultaneously through single close/trip command. 41


TESTING PROCEDURE a) Make connections as shown in the figure-14 below. It is to be ensured that R, Y, B phase marking cables are connected with the proper place in the CB analyser and colour codes are to be maintained for all the three poles of CB. b) Make connections for recording operating timings of Auxiliary contacts. c) Extend power supply to Circuit Breaker Analyzer. d) Give closing command to closing coil of CB and note down the PIR and main contact closing time. Take the print out from the analyser. e) Give tripping command to trip coil-I of CB & note down the main contact tripping time. f) Give tripping command to trip coil-II of CB & note down the main contact closing time. g) Note down the timings for `CO', and `OCO' by giving respective commands. CO command to be given without time delay but 300ms time delay to be given between O and CO operation in testing for OCO. h) To find out opening time of PIR contacts, PIR assembly has to be electrically isolated from Main contacts and then PIR contacts are to be connected to separate digital channels of the Analyzer. EVALUATION OF TEST RESULTS A) CLOSING TIMINGS Closing timings and Discrepancy in operating times of PIR and main contacts should not exceed the permissible limits as specified in the DOC NO: D-5-02-XX-01-03. In any case, main contacts should not close prior to closing of PIR contacts and PIR contacts should not open prior to closing of main contacts. In case, contact bouncing is observed in operating timings for PIR and main contacts, same should be rectified by tightening the cable connections.

Fig. 14 42


B) TRIPPING TIMINGS Trip time and pole discrepancy in operating timings should not exceed beyond permissible value given in Doc. No. D-5-02-XX-01-03. In case of ABB, NGEF and CGL make CBs, while tripping, PIR contacts should not open after opening of main contacts.

C) ‘CO' TIMINGS CO timings should be within permissible limits as specified by different manufacturers. If operating timings of CB poles are not within limits, same may be corrected by: 1. Equalizing the SF6 gas pressure in all the poles 2. Adjusting plunger movement of trip/ close coils 3. Adjustment in operating mechanism 4. Changing of trip/ close coils (if required) It is also important to measure timings of auxiliary contacts from the point of view of variations w.r.t. the main contacts. 3.3.3 DYNAMIC CONTACT RESISTANCE MEASUREMENT (DCRM) CONTACT TRAVEL MEASUREMENT OF EHV CIRCUIT BREAKERS

AND

Test Equipment: 100 Amp. DCRM kit with CB operational analyzer with 10k Hz sampling frequency. Isolation Required a) CB should be in open position. b) Isolator of both sides of CB should be in open position. c) Earth switch of one side of CB should be in open position. Precautions a) There should be no joints in testing leads/cables. b) It should be ensured that whole testing equipment along with testing procedures are available at testing site. Testing must be carried out in presence of testing personnel only. c) Current leads should be connected such that voltage leads are not outside area of current flow. Testing Procedure 1. Follow the standard procedure as given in instruction manual of DCRM kit. 2. The tightness of connections at CB flanges is most important to ensure error free measurement. CB during CO operation generates lot of vibrations and failure of connections during this period can dramatically change the dynamic signature of CB resistance. 3. DCRM signatures should be recorded for CO operation. Open command should be extended after 300 ms from the close command. 4. Clean portions of incoming and outgoing flanges of CB with polish paper to remove paint, oxidation etc, at points where Current clamps are mounted. 43


5. Select this point of connection, as close as possible to the end of porcelain insulator to ensure that minimum resistance is offered by flanges, bolts, terminal connectors etc. 6. It should be ensured that Travel Transducers are properly fitted. 7. Sampling frequency during measurement should be 10 KHz. 8. Reistance, travel, injected current and Trip/ Close coil currents are to be recorded. The variations in the measured resistance versus time will be seen as a finger print for the breaker contacts and can be used as a bench mark for comparing with future measurements on the same breaker. This provides information on the condition of the breaker contacts, driving mechanism, operating levers etc. Dynamic Contact Resistance Measurement for CB healthiness By application of Dynamic Contact Resistance Measurement, condition of arcing contact, main contact, operating levers, driving mechanism can be predicted. If DCRM signature shows vide variations and also there is change in arcing contact insertion time, it indicates erosion of the arcing contacts to main contacts and subsequent failure. Contact Travel Measurement Transducers are attached to the operating rod or interrupting chamber in order to record the contact travel. When CB closes, contact travel is recorded. Contact bounces or any other abnormality is also clearly indicated by the Contact Travel Measurement. If contact travel, contact speed and contact acceleration signature are compared with the original signatures, then it may indicate problems related with the operating mechanism, operating levers, main/ arcing contacts, alignments etc. DCRM along with Contact Travel measurement is useful in monitoring length of Arcing contacts. Erosion of Arcing contacts may lead to commutation failures and current may get transferred to Main contacts. Due to heat of arc, main contacts may get damaged. 3.3.4

OPERATIONAL LOCKOUT CHECKING FOR EHV CIRCUIT BREAKERS

3.3.4.1 TESTING PROCEDURE: A. SF6 GAS PRESSURE LOCKOUT a) LOW PRESSURE ALARM Close Isolation Valve between CB Pole(s) and density monitor. Start releasing SF6 gas from density monitor till the low pressure gas alarm contacts are actuated which is detected by Multimeter. Note down the pressure and temperature at which the contacts get actuated. b) OPERATIONAL LOCKOUT: Continue releasing SF6 gas from isolated zone till the operational lockout Alarm Contacts are actuated which are detected by Multimeter. Note down the pressure and 44


temperature at which the contacts get actuated. This is called operational lockout pressure. B. PNEUMATIC OPERATING SYSTEM LOCKOUT a) COMPRESSOR START/STOP SWITCH Close the isolating valve of CB. Release air into atmosphere from the compressor. Note down the value of pressure at which Compressor starts building up air pressure and pressure at which Compressor stops. b) CB AUTO RECLOSE LOCKOUT Close Isolation valve between pneumatic system and pressure switches. Release air from the isolated zone to atmosphere. Note down pressure at which A/R L/O contacts of pressure switch get actuated which are detected by Multimeter. The leads of the Multimeter should be connected to the contactor where the AR L/O of CB are made. c) CB CLOSING LOCKOUT Release air from the isolated zone to atmosphere. Note down pressure at which CB Closing L/O contacts of pressure switch get actuated which are detected by Multimeter. d) CB OPERATIONAL LOCKOUT Release air from the isolated zone to atmosphere. Note down pressure at which CB Operational L/O contacts of pressure switch get actuated which are detected by Multimeter. e) MECHANICAL CLOSING INTERLOCK ( FOR ABB CBs ONLY) CB should be in closed position. Release air from pneumatic system of CB to atmosphere and observe whether CB poles start opening, if so, note down the pressure at which tie rod starts coming down. In such case the closing interlock is to be opened for inspection and if required, replace the closing interlock. C. HYDRAULIC OPERATING SYSTEM LOCKOUT a) Pump START/STOP By opening pressure release valve, note down the pressure at which Pump starts building up oil pressure and pressure at which pump stops. b) CB AUTO RECLOSE LOCKOUT Close Isolation valve between hydraulic system and pressure switches. Release oil from the isolated zone to oil tank. Note down pressure at which A/R L/O contacts of pressure switch get actuated which are detected by Multimeter. c) CB CLOSING LOCKOUT Release oil from the isolated zone to oil tank. Note down pressure at which CB Closing L/O contacts of pressure switch get actuated which are detected by Multimeter. d) CB OPERATIONAL LOCKOUT Release oil from the isolated zone to container. Note down pressure at which CB Operational L/O contacts of pressure switch get actuated which are detected by Multimeter. 45


3.3.4.2 EVALUATION OF TEST RESULTS A. SF6 GAS PRESSURE LOCKOUT All the SF6 gas pressure switches settings should be checked and corrected with ambient temperature. Settings of SF6 gas pressure switches should be within ± 0.1 bar/ Kg/cm2 of the set value ( after taking into account the temperature correction factor). B. AIR PRESSURE LOCKOUT All the air pressure switches settings should be checked and corrected and should be within ± 0.3 bar/ Kg/cm2 of the set value. C. OIL PRESSURE LOCKOUT All the oil pressure switches settings should be checked and corrected and should be within ± 0.3 bar/ Kg/cm2 of the set value. 3.3.5 MEASUREMENT OF STATIC CONTACT RESISTANCE The Static contact resistance of main circuit of each pole of a circuit breaker is of the order of a few tens of micro ohms. 100 A DC is injected and milli volt drop is measured across each CB contact to compute contact resistance. The values should be within specified limits as given in Doc No. D-5-02-XX-01-01. 3.3.6 CHECKING THE ANTI-PUMPING FEATURE When the breaker is in open position and closing and opening commands are given simultaneously the breaker first closes and then opens, but does not reclose even though the closing command is maintained. 3.3.7 CHECKING THE ANTI-CONDENSATION HEATERS Check the supervisory circuit of the anti-condensation heaters for correct functioning. With the heaters switched ON, measure their current output. 3.3.8

POLE DISCREPANCY RELAY TESTING

Pole Discrepancy is defined as the difference in closing & opening timings of different poles of CB. A. WHEN CB IN OPEN POSITION Closing Command is extended to close one pole, say R-Pole, of CB. After closing R-Pole of CB, this Pole should automatically open after 2.5 seconds ( as per pole discrepancy timer settings). Repeat the test for remaining two poles of CB. B. WHEN CB IN CLOSED POSITION Tripping Command is extended to trip one pole, say R-Pole, of CB. Remaining Y and BPoles of CB should automatically open after 2.5 seconds. Repeat the same test for remaining two poles of CB. 46


C. EVALUATION OF TEST RESULTS Permissible value of pole discrepancy between two poles of CB is 3.33 msec. from system point of view and it should not be confused with the setting of pole discrepancy timer which is generally 1.0 or 2.5 sec. depending on Auto-reclose scheme. 3.3.9

CHECKING THE NITROGEN PRIMING PRESSURE

150

160

170

180

190

200

210

220

230

bar

Close the pressure release valve. Shortly after the oil pump starts, the priming pressure (200 bar at 20°C) in the accumulator can be read. The relationship between the pressure and temperature is indicated in Fig. 15.

P rim in g p re ss u re a s a fu n c tio n o f th e te m p e ra tu re N o m in a l ra tin g a t 2 0 d e g .C : 2 0 0 .1 b a r

Figure - 15 3.4 CHECKS/ TESTS APPLICABLE FOR CVTs 3.4.1

CVT POLARITY, RATIO TEST

CVT polarity is checked in the same manner as for CT, taking care to ensure that the battery is connected to the primary winding. In case of star/star winding configuration care has to be taken to ensure that the primary and secondary neutral points are not connected together. It is necessary to verify that the phase rotation sequence of the 3 phase CVT is correct. The secondary voltage between phases and neutral are measured and then phase rotation meter is connected across the three phase terminal. 3.5 CHECKS/ TESTS APPLICABLE FOR ISOLATORS 3.5.1 MILLI VOLT DROP TESTS The voltage drop across the breaker pole is measured for different values of DC currents. The voltage drop gives a measure of resistance of current carrying part and contacts. 47


The DC current should be equal to or more than 100 A. The resistance of breaker pole should be measured at ambient air temperature. The temperature of specimen/environmental temperature should be recorded. The value of measured resistance should be converted to the value of temperature at which factory test results are taken. Temperature corrected value of resistance should be comparable to the factory value. However, while measuring the contact resistance between various joints/connectors, care should be taken such that the value of contact resistance should be below 5 micro-ohms. 3.5.2 50 OPERATION TESTS 3.6 CHECKS/ TESTS APPLICABLE FOR SURGE ARRESTORS 3.6.1 MEASUREMENT OF THIRD HARMONIC RESISTIVE CURRENT FOR SURGE ARRESTERS Testing Procedure a. Make the connections as per the diagram given below (Fig.16) b. The kit should be properly earthed. c. Clamp On type CT should be placed above the surge monitor to pick up the total leakage current. d. Carryout the measurements as per standard procedure supplied by the test kit manufacturer. e. Note down the system voltage and ambient temperature along with the test current value. f. Avoid measurement during monsoon.

Fig 16 Typical arrangement for THRCM Test EVALUATION OF TEST RESULTS A. ZnO Surge Arrester continuously conducts a small leakage current (Fig.17). The resistive component of this leakage current may increase with time due to different stresses causing ageing and finally cause arrester failure. B.

If Harmonics are present in the system voltage, it affects the value of measured third harmonic current. Compensating device provided to be used to nullify the effect. The value of Third Harmonic Resistive current shall be less than 30 µA.

48


Figure-17 Arrester equivalent circuit

3.7

CHECKS/ TESTS FOR OTHER AREAS/ EQUIPMENTS

3.7.1 EARTH RESISTANCE MEASUREMENT Normally Earth tester is used for measuring (a)

Soil resistivity

(b)

Earth resistance

a.

Prior to the testing of soil resistivity and earth resistance the operation manual of the testing instrument available at site may be referred for and procedures to be adopted for measurement of soil resistivity and earth resistance. A typical earth tester has 4 terminals. C1, P1, C2, P2 and 4 similar electrodes are driven in the ground at equal distances and connected to the instruments in the order of C1, P1 and P2, C2. Then the handle is rotated or button is pressed and the reading of the resistance is read on the scale of the instrument. If R is the resistance measured then the .. Specific Resistivity = 2 π a R Where ‘a’ is the distance between the electrode in ……………. And R is the resistance in ohms measured on the earth tester.

b.

In order to measure earth resistance of the electrode of the substation, it could be connected to C1 and the value of R could be read in the scale with the rotation of the handle of the Insulation tester. This will give the earth resistance. The value as far as possible should be around 1 ohm. To improve the value, water should be spread at the earth pit. 49


3.7.2 PRIMARY CURRENT INJECTION TEST Most protective gear is fed from a current transformer and the supply cable or bus bars and primary current injection testing checks all part of the protection system by injecting the test current through the primary circuit. The detailed circuit diagram for primary injection test is shown in the test format No. CF/bay/03 of page 8. However, based on the schematics approved by the Engg., the circuit diagram may be suitably modified and checked. The primary injection tests can be carried out by means of primary injection test sets. These sets are standard portable sets comprising of Current supply unit, Control unit and Accessories. The test set gives variable output current. The output current can be varied by means of built in auto transformers. The primary injection test set is connected to AC single phase supply (3 phase supply) depending upon the manufacturer’s recommendation. The output terminal can be connected to in the primary circuit of current transformer. The test setup is shown in fig. 18. The primary current can be varied by means of injection test and the secondary current measurement has to be carried out at various points as indicated in the circuit diagram of the reference pages of the primary injection test formats.

P1

O/C

P1

P1

S1

S1

S1

S2

S2

S2

E/F RELAY TEST SOCKET

A

PRIMARY INJECTION KIT

Figure-18 3.7.3

SECONDARY CURRENT INJECTION TEST SETS

The primary test is essential when commissioning and new installation as a test the whole protection system and will detect current transformers connected with incorrect polarity or relays that have been set in the wrong sequence in differential system. Secondary current injection sets are very useful for conducting these tests. The standard secondary current injection test equipment consists of a 50 A current injection set, separate wave form filter unit and a digital counter. The equipment is designed in a portable kit for on site testing of protecting devices, circuit breakers, trip coils, motor overloads, and similar apparatus. The filter unit should be used when testing saturating core type relays to ensure that the test current 50


has a substantially sinusoidal waveform. The typical test setup is shown in the figure 19. Details of the testing will be elaborated in the relay testing.

INSULATING TEST PLUGS P1

P1

P1

S1

INJECT CURRENT HERE

S2 A1 A2

LOAD

Figure - 19

51


4.0 CHECKS/ TESTS FOR CONTROL & PROTECTION Various checks/ tests for Control & Protection system are provided in format Sl. No. CF/C&P/10/R-2 DATED 01/08/07 attached with this document. For detailed description of the tests, kindly refer to the following documents: a) Testing procedure for Transmission system protection schemes (Vol-I), Doc No: D-2-03-20-01 Rev-1 b) Testing procedure for Transmission system protection schemes (Vol-II), Doc No: D-2-03-20-02 Rev-0

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PRE Commissioning Procedures Tr Rev01 PGCIL

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